More middle-aged Americans are killing themselves, and the economy may be the reason, according to a government report.

The annual suicide rate of people 35 to 64 years old rose 28 percent from 1999 to 2010, more than any other age group, the U.S. Centers for Disease Control and Prevention said in the report. The working-age group probably is more affected by the economic downturn in the past half-decade than the young or old, and that may be driving suicide rates higher.

“The suicide rate started accelerating in 2008, 2009 and 2010 — someone might still be working, but their house is underwater, or they’re working but they’re working part-time,” Eric Caine, the director of the CDC’s Injury Control Research Center for Suicide Prevention, said by telephone. “These things ripple into families. There’s an economic stress.”

The increase in middle-age suicides puts a new focus on that group, as opposed to young or old people who typically have been seen as higher risk, the Atlanta-based CDC said.

In addition to the economic downturn and related financial crisis, the growing availability of prescription opioid drugs — such as oxycodone and hydrocodone — may be a contributing factor.

Researchers have only recently begun focusing on society- level causes of suicide, Caine said. “

‘‘Only in the last 10 or 15 years has there been a lot of attention swinging back to the macro forces,’’ Caine said. ‘‘There hasn’t been the kind of research to dig under what’s really a superficial view.”

The suicide rate among the middle-aged group was 17.6 per 100,000 people in 2010, an increase from 13.7 per 100,000 in 1999, the CDC said.

Public Awareness

Hanging was the fastest growing method of killing oneself, while use of guns was the most common method, the agency said.

The suicide trends suggest “a need for increased public awareness of suicide risk factors and research of potential suicide prevention strategies,” the CDC said.

Men are most likely to kill themselves with a gun or suffocation. Women are most likely to use a gun, or poison themselves, according to the report.